Polyester fibers are widely used for clothing and industrial materials because of their various excellent mechanical and other properties. Among them, polyethylene terephthalate (PET) is a typical polyester used for general purposes.
In recent years, a high speed spinning method for obtaining practical PET fibers in one step at a high take-up speed of 5000 m/min or more without drawing has been industrially adopted. Since the productivity in the step of spinning greatly depends on the quantity discharged per unit time, a higher spinning speed can achieve a higher productivity in the one-step method. However, for example, PET fibers show practically preferable mechanical properties when they are spun at a high speed of 6000 to 7000 m/min, but at higher speed, the strength and elongation of the fibers are lowered. Therefore, there is a limit to the spinning speed which can sufficiently exhibit the effect of productivity improvement.
In this regard, it has been proposed to blend a small amount of a polymer incompatible with the matrix polymer for spinning. For example, Japanese Patent Laid-Open Nos. 58-98414 and 60-209015 disclose spinning methods for controlling the molecular orientation by adding 0.1 to 10 wt % of a polymer incompatible with the matrix polymer. Furthermore, Japanese Patent Laid-Open No. 57-11211 discloses a method of adding a liquid crystal polymer. Moreover, Japanese Patent Laid-Open Nos. 56-91013, 57-47912 and 62-21817 disclose methods for controlling the molecular orientation by adding a small amount of a polyolefin based polymer to a polyester.
However, in these methods, the added polymer exerts adverse influence even though the molecular orientation is controlled. For example, when a polymer having low softening temperature such as polystyrene is added, the added polymer existing in the surface layer of fibers may adhere to each other during the falsetwist texturing under a high temperature. In addition, the coloring of dyed fibers may be poor. Moreover, it is very difficult to homogeneously blend a small amount of a different polymer with a polyester. The lack of homogeneity in the blend may cause frequent fiber breakage and the fabrics may not be sufficient in color uniformity after dyeing.
In applications for clothing, falsetwist textured yarns are usually used to provide moderate bulk to a fabric. Recently, draw texturing has become a common process, where highly oriented undrawn fibers, for example, a so-called partially oriented yarn (POY) is one of them, are simultaneously drawn and falsetwisted.
To improve the process stability and processing speed in the draw texturing process, the ballooning of the yarn along the thread line must be stabilized. It is known that this purpose can be achieved at a higher twist tension, and/or a higher drawing ratio and the use of more highly oriented fibers. However, if the drawing ratio is increased to an excessively high value or if excessively highly oriented fibers are used, fluff and frequent fiber breaking occur, thereby lowering the quality of the textured yarns obtained, and disadvantageously inconveniencing the operation. Therefore, in general, the upper limit of the spinning speed for the undrawn yarn which can be textured with a moderate twist tension is about 4000 m/min.
Furthermore, in the melt spinning of a polyester, it is known that at a higher than critical spinning speed, neck-like deformation occurs on the spinning line, to cause orientation and crystallization, and that fibers similar to the conventional drawn fibers can be obtained. For example, it is well known that the critical spinning speed of PET is about 4000 to 5000 m/min. Since the fibers obtained at higher than the critical spinning speed are not the so-called POY, they cannot be processed by draw texturing under a draw ratio of, for example, from 1.2 to 2.0, even though they can be textured without being drawn.
The draw texturing process has a feature that highly oriented undrawn yarn with a large denier corresponding to the drawing ratio can be spun at a high speed. Therefore, a high productivity in spinning can be realized. Heretofore, polyester fibers spun at a spinning speed higher than 4000 m/min could not be stably processed by draw texturing.
The conventional POY is little crystallized as can be seen from its boil off shrinkage of higher than 50% and the absence of the wide angle X-ray diffraction peak, which would have indicated the presence of a crystallization of the polyester. For this reason, the fiber structure is unstable and changes depending upon storage conditions. The storage time of POY must be controlled to be constant to keep uniformity in dyeing. Furthermore, since the structural change of the fibers during storage is different between the inner portion and the outer portion of each package, the quality is uneven disadvantageously. Therefore, to keep uniform quality, heat treatment at a certain elevated temperature must be needed.
One of the objects of the present invention is to provide highly oriented undrawn polyester fibers which at most exhibit only a small change in fiber structure during storage, which, due to their stable structure, have good processability in draw texturing, and are capable of contributing to the improvement of productivity. The other object of the present invention is to provide highly oriented undrawn polyester fibers capable of contributing to the improvement of productivity by increase of output. Another object of the present invention is to provide a falsetwist textured yarn obtained from said highly oriented undrawn polyester fibers, and a process for producing the same.